An electrochromic material undergoes a reversible color change upon the adsorption and desorption of small cations. This property can be exploited to fabricate a device that changes color upon the application of a voltage potential.
The typical electrochromic device comprises an electrochromic layer and an ion-storage layer sandwiched between two electrically conductive substrates, at least one of which is transparent. Optionally, the electrochromic layer and the ion-storage layer can be separated by an ion-conducting layer. The ion-storage layer can optionally be replaced by the ion-conducting layer. Optical properties of the electrochromic device change when ions (e.g., hydrogen ions or lithium ions) intercalated within the structure of the ion-storage layer are removed and interposed with the structure of the electrochromic material in response to an electric potential applied to the electrically conductive substrates. Ions are removed and returned to the ion-storage layer by reversing the polarity of the applied potential, thereby returning the electrochromic device to its original optical state.
The electrochromic layer and the ion-storage layer are similar in that they both adsorb and desorb mobile ions in response to an applied electric field. A simple model for understanding electrochromic devices assumes that the electrochromic layer colors and clears during ion adsorption/desorption, while the ion-conducting layer and ion-storage layer remain transparent. However, practical electrochromic devices can be made if the ion-storage layer colors as well. For example, if the electrochromic layer cycles from clear to color upon ion adsorption (e.g. tungsten oxide), and the ion-storage layer cycles from clear to color upon ion desorption (e.g. nickel oxide), the overall devices will cycle from clear to color. If the electrochromic layer cycles from clear to blue upon ion adsorption (e.g. tungsten oxide), and the ion-storage layer cycles from clear to yellow upon ion adsorption (e.g. vanadium oxide), the overall device will cycle from blue to yellow. Numerous combinations are possible.
Furthermore, if the ion-conducting layer is opaque and the electrochromic layer cycles from clear to blue, the entire device will cycle from blue to the color of the ion-conducting electrolyte layer, regardless of the coloration of the ion-storage layer.
The construction of an electrochromic device typically involves coating electrochromic material onto a transparent, conductive substrate. If the transparent, electrically conductive substrate comprises glass, there are several proven coating methods available. These include evaporation deposition (U.S. Pat. No. 5,598,293) and electro-deposition (U.S. Pat. No. 5,470,673). Of particular advantage and commercially available utility is the coating of a transition metal oxide from an alcoholic solution (U.S. Pat. No. 4,855,161), followed by heating in excess of 200° C. Electrochromic devices produced on rigid glass substrates are handled and assembled individually.
An electrochromic device comprising flexible plastic substrates would have advantages over electrochromic devices comprising glass substrates. These advantages include light weight, durability, shapability, low cost and ease of manufacture. It is particularly desirable that flexible electrochromic devices could be manufactured at high speed on conventional coating/lamination equipment in large rolls, and cut to fit as needed. In order to achieve this goal, however, it is necessary for either the electrochromic layer, the ion-conducting layer, or the ion-storage layer to act as an adhesive for reliable assembly. The present invention focuses on the ion-conducting layer to play this role as an adhesive.
Typically, the adhesive properties of polymers that are known as good adhesives, such as polyalkylacrylates or polyvinylbutyral, deteriorate rapidly when plasticizer and salt are added to make them ion conducting. The ionic conductivity of polymers that are known good ionic conductors, such as perfluorosulfonated anionic polyelectrolyte, deteriorates when they are blended with polymers to make them adhesive. Another approach is to coat an ion-conducting layer containing monomers and/or oligomers and curing these species as a post-processing step to achieve adhesion. However, this involves a complex extra manufacturing step, and it runs the risk of embrittling the flexible device.
There has been extensive effort to provide an ion-conducting layer for lithium batteries, a field that shares common chemistry with electrochromic devices. Many of these low molecular weight polymers have a relatively low dielectric constant when compared to their liquid solvent counterpart, and thus limit the number of charge carriers in the plasticized polymer (U.S. Pat. No. 6,645,675). In an effort to overcome this hindrance, high dielectric constant liquid organic solvents such as ethylene carbonate (EC) and propylene carbonate (PC) have been incorporated in the host polymer, both to increase the number of charge carriers and further increase the room temperature conductivity of the polymer (U.S. Pat. No. 6,828,065). The use of these organic solvents to plasticize polymers such as polyvinylacetal, polyacrylonitrile, polyvinylacetate and hexafluoropropenevinylidene fluoride copolymer is known. However, the mechanical properties of these polymers were so inadequate that they had to be supported on porous matrices. Polyvinylidene fluoride (PVDF) and polyacrylonitrile (PAN) were previously evaluated and have also been doped with a variety of liquid polar solvents, yielding room temperature conductivities as high as 10−3 S/cm. Subsequently, PVDF has been the subject of further study (U.S. Pat. No. 5,296,318). However, these materials were developed for batteries and are not good adhesives for flexible electrochromic devices.
EP 1056097 describes polyacrylate, polystyrene, polyvinylbutyral, polyurethane, polyvinylacetate, polyvinylchloride and polycarbonate as suitable polymer binders for the ion-conducting layer in rigid electrochromic devices. U.S. Pat. No. 6,995,891 cites these same polymeric binders. Nevertheless, with these binders, adhesion is achieved by laminating the devices individually after putting a bead of non-ion-conducting polyvinylbutyral around the device perimeter. As an example for this class, Example 1 illustrates that a polyacrylate binder formulated for ionic conductivity does not function well as an adhesive in an electrochromic device.
U.S. Pat. No. 5,337,184 describes curing the solid electrolyte in place utilizing a polymeric adduct having an acrylic backbone and polyether side chains made by reacting a hydroxyl-functional acrylic co-polymer and a polyethermonoisocyanate. However, this is a complex, extra manufacturing step, with a resulting solid electrolyte that may be too rigid for a flexible device.
Glass electrochromic devices have not achieved broad commercial acceptance in architectural, automotive or eyewear applications, due to practical limitations. First, glass electrochromic devices can be prohibitively expensive to manufacture. Second, glass electrochromic devices cannot function over the decades required for architectural and automotive applications; with each cycle, an electrochromic device suffers a minute but cumulative deterioration in performance, due to the accumulation of an irreversible colored “bronze” and trapped gas. Third, glass electrochromic devices are too heavy for eyewear applications and can also shatter to dangerous shards upon impact of a foreign object.
Plastic electrochromic devices address these limitations. Manufacturing costs are controlled by low capital requirements and high throughput. For example, in accordance with the present invention, an electrochromic layer, an ion-conducting layer, and an ion-storage layer could be coated at a rapid rate on a continuous wide web of electrically conductive polyethylene terephthalate film. The flexible electrochromic device could then be laminated together using the adhesive ion-conducting layer described herein. If this laminated film is applied to architectural and automotive glazing, the film could be replaced if its performance deteriorates over time. If the laminated film is applied to polycarbonate, or if the electrochromic device included polycarbonate coated directly, the electrochromic device would be light and safe enough for eyewear.